Since NOx in a combustion waste gas exhausted from industrial plants, cars, etc. is a material causing photochemical smog, development of a method of removing it is an important and urgent social problem from the standpoint of protecting the environment. It is particularly difficult to remove NO of NOx gases and various methods have been proposed to solve this problem up to the present time. For example, a catalytic reduction method has been proposed and developed, as an effective means, but this method needs a reducing agent such as ammonia, hydrogen, carbon monoxide, etc. and a special apparatus for recovering or decomposing the unreacted reducing agent. On the other hand, a catalytic decomposition method is a method comprising only passing a waste gas through a catalyst bed to decompose NOx into nitrogen and oxygen without need of a special additive. This method is very simple and accordingly, it is most preferable. According to the former studies, it is found that Pt, CuO and Co.sub.3 O.sub.4 have NO decomposing activity, but they tend to be subject to poisoning with oxygen as a decomposition product and cannot be a practical catalyst [Toshio Uchijima, "Hyomen (Surface)" Vol. 18, No. 3 (1980), page 132].
Furthermore, harmful materials such as NOx, CO, HC, etc. which are considered to cause photochemical smog are contained in combustion waste gases exhausted from gasoline engines of cars, diesel engines of buses, tracks, etc. and it is an important and urgent social problem to develop a method of removing them from the standpoint of protecting the environment.
For the removal of NOx in a waste gas, there are adsorption methods, oxidation and absorption methods, catalytic reduction methods and the like, but the catalytic reduction methods have been considered advantageous from the economical and technical point of view because of no need of after-treatments. This catalytic reduction method is a method comprising passing a waste gas through a catalyst bed in the presence of a reducing gas and thereby converting NOx into unharmful nitrogen, which is classified into two methods depending on the variety of the reducing agents.
That is, these methods are the so-called non-selective reduction method wherein reduction of a waste gas is carried out by adding a reducing gas such as hydrogen, carbon monoxide, hydrocarbons, etc. and contacting with a catalyst and the so-called selective reduction method wherein the reduction is carried out by adding a reducing gas such as ammonia, etc. and contacting with a catalyst. The former method has the disadvantage that the reducing agent is reacted with oxygen jointly present in the gas and then the removal reaction of NOx proceeds, resulting in need of the reducing agent in a large amount, but in a case where a waste gas such as exhausted from internal combustion engines of cars previously contains a reducing agent such as carbon monoxide, hydrocarbons, etc. in an amount of at least equimolar to oxygen, the non-selective reduction method is more advantageous for removing NOx in the waste gas. As a catalyst for the removal of NOx in waste gases from cars, it is preferable to use one for the non-selective reduction reaction.
When no catalyst is used, a waste gas from a car has a gas composition as shown in FIG. 1, and when a three way catalyst capable of simultaneously removing NOx, CO and HC is used, the gas composition is as shown in FIG. 2, and the three components of NOx, CO and HC are removed approximately at the theoretical fuel ratio. The engine combustion waste gas at the theoretical air fuel ratio has the following composition:
CO: 0.3.about.1%; NO: 0.05.about.0.15%; H.sub.2 O: about 13% PA1 H.sub.2 : 0.1.about.0.3%; HC: 0.03.about.0.08%; SO.sub.2 : about 0.002% PA1 O.sub.2 : 0.2.about.0.5%; CO.sub.2 : about 12%. PA1 CO: 0.02.about.0.1%; NO: 0.02.about.0.1%; HC: 0.02.about.0.1%; O.sub.2 : 5.about.15%.
The chemical reactions, in which NOx, CO and HC participate in the above described waste gas, are as follows:
HC includes a wide range of C.sub.1 to C.sub.7, i.e. methane (CH.sub.4) to toluene (C.sub.7 H.sub.8), and accordingly, in the following reaction schemes, HC is represented by ethylene (C.sub.2 H.sub.4). EQU CO+1/2O.sub.2 .fwdarw.CO.sub.2 ( 1) EQU C.sub.2 H.sub.4 +3O.sub.2 .fwdarw.2CO.sub.2 +2H.sub.2 O (2) EQU 2NO+2CO.fwdarw.2CO.sub.2 +N.sub.2 ( 3) EQU 6NO+C.sub.2 H.sub.4 .fwdarw.2CO.sub.2 +2H.sub.2 O+3N.sub.2 ( 4) EQU 2NO+2H.sub.2 .fwdarw.2H.sub.2 O+N.sub.2 ( 5) EQU CO+H.sub.2 O.fwdarw.CO.sub.2 +H.sub.2 ( 6) EQU 2NO.fwdarw.N.sub.2 +O.sub.2 ( 7).
It has been proposed to subject silica gel or zeolite to ion-exchange with Cu as a substitute of the above described catalyst, but the resulting catalysts have the following problem:
(1) The catalyst obtained by supporting copper ion on silica gel by an ion exchange method has a considerably high initial activity, but the activity is rapidly lowered with the passage of time.
(2) The catalyst obtained by subjecting a Y-type zeolite or mordenite to ion-exchange with copper has a low decomposition activity in the presence of oxygen.
(3) The catalyst obtained by subjecting a ZSM-5 type zeolite to ion-exchange with copper has a problem that the decomposition activity of NO is high, but the selectivity in the conversion of NO into N.sub.2 (2NO.fwdarw.N.sub.2 +O.sub.2) is low (Japanese Patent Laid-Open Publication No. 125250/1985).
When using the three way catalyst having been put to practical use for the purpose of purifying a waste gas from a car engine, oxygen is consumed by the oxidation reactions of the above described reactions (1) and (2) and the reduction reactions of NO of the above described reactions (3) to (5) proceed under such a state that the oxygen concentration is considerably lowered. Therefore, the three way catalyst is effective only near the theoretical air fuel ratio (14.6) and it is difficult to reduce NOx in a high air fuel ratio zone in which the rate of fuel consumption can be decreased ["Shokubai (Catalyst)" Vol. 29, No. 7, 1987, p 598-609].
Since the air fuel ratio of a diesel engine is in the range of 20 to 80 and the oxygen concentration is high, on the other hand, removal of NOx by a catalyst is difficult at the present time ["Shokubai Koza (Catalyst Lecture)" 7, Kihon Kogyo Hanno (Fundamental Commercial Reactions) by Shokubai Gakkai, published by Kodansha KK].
An example of the composition of a waste gas from a diesel engine is as follows:
The inventors have made various efforts to solve the above described problems of the prior art and consequently, have found that the crystalline silicate having the specified composition and crystalline structure and having been subjected to ion-exchange with copper has not only a very high activity as a catalyst of catalytically decomposing NO, but also a high selectivity for the conversion of NO into N.sub.2 and exhibits a stable activity even in the presence of oxygen and SOx, and when using this catalyst, NOx, CO and HC are decreased near the theoretical air fuel ratio and in a range of higher than the theoretical air fuel ratio, i.e. in the presence of oxygen, that is to say, the above described reactions (1) to (7) take place. The present invention is based on this finding.